How can i add specularity, shadows and decals on top of a Blender Cycles bake
Switch to Blender Render or Blender Game, import your baked texture and set the emit to 1.0 to get the full lighting information. Optionally you can then import a roughness texture and set it as specular color to create some effects on the surfaces.

Add point light where light are coming from, inside a room add them on each window. Change the color to match the color of the exterior (sky, green garden, other room). Uncheck Diffuse so it will not emit light and leave the Specular checker.

Now adjust the specular intensity of your material and the hardness.



For the shadow add a sun, keep the same sun orientation that you had used for the Cycles baking and enable shadow, check Only Shadow but keep the Diffuse and Specular checked. Adjust the light intensity to contrast the shadows, set the Buffer Type to variance (to get smooth shadow borders) and adjust the settings. Use the Shadow Box to only include your level to maximize the performances or parent it to the camera and reduce the shadow box so you get shadows on close objects only.

For decays add a new texture completely transparent, you can use an high resolution, then paint or project images on your 3D model using the stencil mode. Blend the image in Multiply mode to keep the lighting informations.



You can then add post-processing filter like Bloom in the logic editor.